So all day long the noise of battle roll'd
Among the mountains by the winter sea,
Until King Arthur's table, man by man,
Had fallen in Lyonesse about their lord.
Tennyson

The earliest literary mention of Lyonesse
may be in Beroul where is is called "Loenois". 11th November, 1099,
is one of the traditional Cornish dates for the inundation of Lyonesse. The
first mentions of Lyonesse in Cornwall seem
to be by Carew and Camden in Camden's 1586 edition of
"Britannia". Among other things Carew says of Lyonesse (in English translation)".

...about the middle way between Land's End and Scilly, there are rocks call'd in
Cornish Lethas, by the English Seven-stones; and the Cornish call that place Tregva,
i.e. a dwelling, where it has been reported that windows and other such
stuff have been taken up with hooks (for that is the best place for
fishing)".

Another claim is that Lyonesse originally referred to Lothian in Scotland
which is attested as (L(a)odone(i)o 1096 Laudonie 1126 Loeneis 1158 Leudonia
1164; variously Loida (Symeon of Durham) Laodonia (Aelred) Lodene (O. Eng. Chron )
Lowthyan (Wyntoun). In Gottfried's Tristan, we have the reference to

'His true name was Rivalin, his byname Canelengres. Many affirm and
believe that this same lord was a Lohnoisan and king of the land of Lohnois...' (Hatto p46)

Whatever its origin, the idea of Lyonesse is a powerful one. It has the
pull of Atlantis. It is the land where Tristan was born and
Arthur died, a land of faery consigned to the deep by Merlin.
It has inspired poets, musicians and artists throughout the ages.